1. Field of the Invention
The present invention relates to a method of forming tungsten employed as a wiring or a buried layer in a contact hole.
2. Description of the Prior Art
With the progress of high integration of a DRAM (Dynamic Random Access Memory) and a logic LSI in recent years, a diameter of a contact hole formed in an interlayer insulating film to connect an overlying wiring layer and an underlying wiring layer is reduced, and also a width of a buried wiring layer in an insulating film is miniaturized. Thus, aspect ratios of the contact hole and the wiring layer are increasing.
Using effectively characteristics of tungsten which has lower resistance than silicide or silicon and is proof against high temperature heat treatment, a tungsten wiring layer is employed as one of wiring layers of the DRAM and others. For example, the tungsten wiring layer is employed as a word line which contacts electrically a gate electrode, which is formed of a laminated structure (polycide) of polysilicon and tungsten silicide, a wiring layer which contacts electrically an N.sup.+ -type silicon layer and a P.sup.+ -type silicon layer of peripheral circuits, and the like.
Normally, the tungsten wiring layer is grown by reducing tungsten halide such as WF.sub.6, etc. by using a hydrogen gas or a hydride gas such as silane (SiH.sub.4), disilane (Si.sub.2 H.sub.6), phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6), etc. and a hydrogen mixed gas or a hydride mixed gas. However, when the tungsten film is formed by reduction action using hydrogen or hydride, normally its resistivity is relatively high like 15 .mu..OMEGA.cm. Thus, an electric resistance of the tungsten wiring layer employing such tungsten film becomes high.
Therefore, since the resistivity of the tungsten film is low like 8 to 10 .mu..OMEGA.cm if the film is formed by executing the reduction using a gas into which diborane in hydride is mixed, the resistance of the tungsten wiring layer can be relatively reduced. Thus, the tungsten wiring layer formed by such growth method is being observed with interest. Such technology is set forth in Patent Application Publication (KOKAI) Hei 4-74865, for example.
The diborane must be employed in such technology. Nevertheless, actually it is possible to grow the tungsten film by mixing a very small amount of diborane into hydrogen, etc. and thus it is not always needed to employ diborane as a main component. However, for convenience of explanation, a reduction method which employs a hydrogen gas or a hydride gas into which diborane is mixed, a mixed gas containing hydrogen and diborane, or a mixed gas containing hydride and diborane is called a "diborane reduction method" in the following description. Also, sometimes a reduction method which employs the hydrogen gas or the hydride gas, a hydrogen mixed gas, or a hydride mixed gas, into which diborane is not mixed, is called a "hydrogen reduction method".
FIG. 1 is a sectional view showing an example of a semiconductor device in which the tungsten film which is grown by the diborane reduction method is employed as a wiring layer which is brought into contact with source/drain diffusion layers. In FIG. 1, a reference 101 denotes a silicon substrate; 102, an element isolation LOCOS oxide film; 103, a gate oxide film; 104, a gate electrode; 105 and 106, low impurity concentration diffusion layers; 107 and 108, high impurity concentration diffusion layers; 109, an insulating film for covering a periphery of the gate; 110, an interlayer insulating film; 111, a double-layered film formed of TiN and Ti; and 112, a tungsten film grown by the diborane reduction method. The diffusion layers 105, 107 and the diffusion layers 106, 108 serve alternatively as a source region and a drain region in an LDD (Lightly Doped Drain) structure. The double-layered film 111 formed of TiN and Ti is formed to improve adhesiveness between the tungsten film 112 and the interlayer insulating film 110. An upper layer of the double-layered film 111 is formed of TiN to improve the adhesiveness, while a lower layer thereof is formed of Ti to contact the diffusion layer 106. The insulating film 109 covering a periphery of the gate electrode is employed in the step of forming an MOS transistor with the LDD structure, or the step of opening the contact hole in the interlayer insulating film 110 by a self-alignment contact method.
In the meanwhile, the interlayer insulating film 110 is employed to isolate and separate the diffusion layer 106 and the upper wiring layer 112 and also the gate electrode 104 and the upper wiring layer 112 respectively. In addition, in the case of the dynamic RAM, an insulating layer which separates a capacitor structure (not shown) of a stacked capacitor from the upper wiring layer 112 is stacked on the interlayer insulating film 110. In such case, a thickness of the interlayer insulating film 110 must be formed thicker by the stacked capacitor structure.
The interlayer insulating film having a multi-layered structure is also employed in the logic LSI as well as the memory semiconductor device. Also, it is not preferable to thin the interlayer insulating film because its parasitic capacitance is increased, so that the interlayer insulating film needs a thickness to some extent.
On the contrary, reduction in a diameter of the contact hole is requested according to miniaturization of the device. Therefore, there is such a tendency that an aspect ratio (b/a), which is a ratio of a contact hole depth b to a contact hole diameter a, is increased.
FIG. 2 is a view showing a shape of the tungsten film formed when the aspect ratio is increased. FIG. 2 is a view showing schematically a SEM photograph after a sectional shape obtained when the tungsten film 112 is formed in a hole, which has a diameter of 0.25 .mu.m and a depth of 2.0 .mu.m, by using the diborane reduction method is taken by the SEM photograph. In FIG. 2, the adhesive layer 111 is provided.
As can be seen from FIG. 2, in the case of the high aspect ratio such that the aspect ratio is about 8 (2.0/0.25), the tungsten film is seldom formed on a bottom portion and a peripheral portion of the contact hole.
Accordingly, in the structure shown in FIG.1, when the aspect ratio is increased because the diameter of the contact hole on the diffusion layer 106 is reduced with respect to the thickness of the interlayer insulating film 110, the tungsten film 112 cannot contact the diffusion layer 106.
Such phenomenon appears in the tungsten film which is grown by the diborane reduction method remarkably rather than the tungsten film which is grown by the hydrogen reduction method. In other words, the tungsten film which is formed by the diborane reduction method has lower resistivity but has poorer coverage on the stepped portion than the tungsten film which is grown by the reduction action using other hydride or hydrogen. Hence, if growth of the tungsten film employing the diborane reduction method is applied to the minute contact hole which has the high aspect ratio, the tungsten film is difficult to grow on the side wall and the bottom surface of the contact hole and thus there is a high possibility that disconnection of the wiring is caused.
If the tungsten film which is formed by using other hydride or hydrogen in place of diborane as the reduction gas is employed, such disconnection of the wiring can be avoided. However, as described foregoing, since the tungsten film which is grown by the hydrogen reduction method has the high resistivity of 15 .mu..OMEGA.cm, the resistance of tungsten is enhanced even when the tungsten film can contact to the underlying conductive layer. As a result, it is impossible to use the tungsten film as the wiring layer as it is.
In contrast, it may be thought of that, in the tungsten film grown by the hydrogen reduction method, a film thickness is made thick to have the resistance equivalent to that of the tungsten film grown by the diborane reduction method. However, such problems have arisen that not only throughput is reduced because it takes a lot of time to etch the tungsten film, but also the wiring layer cannot be formed with good precision because a retreat amount of the resist is increased during etching.